It is well established that polyethylene copolymers (for example, ethylene alpha-olefin copolymers) may be prepared with a variety of catalysts, such as Ziegler-Natta catalysts, chromium based catalysts, metallocene catalysts, non-metallocene bulky ligand transition metal catalysts or combinations thereof.
Single site catalysts, such as metallocene catalysts, may be used to prepare polyethylene copolymers that are relatively homogeneous. In contrast to traditional Ziegler-Nana catalyst compositions, single site catalyst compositions, such as metallocene catalysts, are catalytic compounds in which each catalyst molecule contains one or only a few types of polymerization sites. Single site catalysts often produce polyethylene copolymers that have a narrow molecular weight distribution. Further, a single site catalyst will often incorporate comonomer among the molecules of the polyethylene copolymer at a relatively uniform rate. The molecular weight distribution and the amount of comonomer incorporation can be used to determine a composition distribution.
The composition distribution of a polyethylene copolymer refers to the distribution of comonomer, which forms short chain branches, among the molecules that comprise the polyethylene polymer. When the amount of short chain branches varies among the polyethylene molecules, the resin is said to have a “broad” composition distribution. When the amount of comonomer per 1000 carbons is similar among the polyethylene molecules of different chain lengths, the composition distribution is said to be “narrow”.
It is generally known in the art that a polyolefin's composition distribution is largely dictated by the type of catalyst used and is typically invariable for a given catalyst system. Ziegler-Natta catalysts and chromium based catalysts produce resins with broad composition distributions (BCD), whereas metallocene catalysts normally produce resins with narrow composition distributions (NCD).
Resins having a broad orthogonal composition distribution (BOCD) in which the comonomer is incorporated predominantly in the high molecular weight chains can lead to improved physical properties, for example toughness properties and environmental stress crack resistance (ESCR).
Two of the most important analytical characterization tools for polyethylene copolymers are gel permeation chromatography (GPC) for molecular weight and temperature rising elution fractionation (TREF) for composition. The key metrics of number average molecular weight (Mn), weight average molecular weight (Mw) and z-average molecular weight (Mz) may be calculated from GPC according to the following equations:Mn=ΣNiMi/ΣNi Mw=ΣNiMi2/ΣNiM1 Mz=ΣNiMi3/ΣNiM1Mi2 where Ni is the number of copolymers having molecular weight Mi.
GPC data are typically represented by plotting the mass fraction (NiMi) on the y-axis, and molecular weight (Mi) on the x-axis. Ni may then be obtained by dividing NiMi by Mi, so that the above equations are easily solved. The availability of such metrics from GPC measurements makes it straightforward to correlate the metrics with polymer properties and performance.
In contrast, TREF data is typically presented without any accompanying metrics. This makes is difficult to correlate TREF data with polymer performance. A typical TREF curve is displayed by plotting mass fraction (NiMi) on the y-axis, and elution temperature on the x-axis. However, it is more useful and informative to convert the temperature axis to a composition measure, such as weight fraction of comonomer. This may be achieved by using a calibration curve, or by directly measuring the composition of the effluent by infra-red or NMR spectroscopy. In this way, the data can be represented by Ci, which is the weight fraction of comonomer in a polymer chain.
It would be desirable to provide polyethylene copolymers having a broad orthogonal composition distribution, that is, with comonomer predominantly incorporated into the high molecular weight chains, and further desirable to provide metrics that may describe the comonomer distribution in such copolymers.